In the fabrication from thermoplastic materials of massive, thick-walled molded parts with a smooth surface, it is known, in order to prevent of cave-in spots, which occur during cooling of an injected compact plastic mass, to provide the inside of the molded part with a plastic mass, containing a foaming agent. By means of this compact mass, the outer skinforming plastic mass is pressed constantly against the inside surface of the mold during the cooling procedure.
It is known, for example, from German patent No. 1,778,457 that one can inject into a mold, a charge of non-foaming mass which does not fill the mold and which is a compact thermoplastic mass; then, before hardening of the center, one can inject a second charge of thermoplastic containing the foaming agent into the center of the first charge. In that way, the mass of the second charge (containing the foaming agent) presses equally on all sides towards the inside of the mold. Thereafter, a small amount of plastic mass may be injected into the mold.
By extending this procedure, it has been made known through German petty patent No. 2 241 002 to inject simultaneously a charge with and without the mass containing the foaming agent after the injection of the first charge to form a smooth surface for the molded part. The plastic mass containing the foaming agent flows into the mold through a central jet, and the compact material forming the smooth outer surface enters the mold through a ring jet surrounding the center jet.
The apparatus for executing this procedure is normally equipped with two separate extruders. In one of the extruders prepares the plastic containing the foaming agent and in the other extruder is prepared the dense plastic mass. The liquid forming the gaseous foaming agent is already mixed into the plastic mass within the extruder. The use of two extruders makes the apparatus quite complicated and expensive.
When a dense plastic mass and a mass containing the foaming agent is injected into the mold one after the other, then it is possible to use only the extruder which delivers dense plastic mass. Shortly after the start of the injection procedure, a predetermined amount of dense plastic is already within the mold cavity. The foaming agent is added to the stream of compact flowing mass in the portion between the extruder and the mold cavity. The disadvantage exists, however, that the total cross-section of the flow canal is filled with the plastic mass containing the foaming agent and, during flushing with compact plastic, the plastic containing foaming agent is not completely removed. This will influence the surface quality of the next molded part. In addition, a thorough mixing of the compact plastic mass with the foaming agent becomes difficult and an exact metering of the plastic mass containing the foaming agent is not possible.
It is, therefore, an outstanding object of the invention to provide apparatus for the discontinuous fabrication of the molded parts made from thermoplastics, which parts have a smooth surface and a porous core, whereby the use of only one extruder allows the simultaneous injection of dense plastic and plastic containing forming agent, thus permitting a good mixing as well as exact metering of the plastic containing the foaming agent.
With this and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.